Corrugated thermoplastic tube may be made an apparatus of the travelling mold tunnel type with vacuum forming is disclosed, for example, in U.S. Pat. No. 4,226,580 issued Oct. 7th, 1980 to Gerd P. H. Lupke and Manfred A. A. Lupke, U.S. Pat. No. 4,319,872 issued Mar. 16, 1982 to Manfred A. A. Lupke, U.S. Pat. No. 3,538,209 issued Nov. 3, 1970 to W. Hegler, and U.S. Pat. No. 5,002,478 to Manfred A. A. Lupke on Mar. 26, 1991. Numerous other patents generally concern travelling mold type apparatus for forming profiled tube. The patents referred to are intended to be merely exemplary of the large number in the field.
A travelling mold tunnel may often be utilized for molding tube of different diameters by substitution of the mold blocks or by the use of interchangeable mold blocks as disclosed in U.S. Pat. No. 4,325,685 issued on Apr. 20, 1982 to Manfred A. A. Lupke. However, a necessary criterion for such tunnels is that they be of sufficient size and power to accommodate the largest mold blocks which it is expected to use. Such mold tunnels are large and may be uneconomic in use for molding tube of very small diameter using small mold blocks.
One reason for the reduced efficiency in the molding of small diameter tube is due to the utilisation of the cooling capacity of the machine. This is measured in terms of the weight of tube which can be produced on the machine during a given period. If large mold blocks are used in a travelling mold tunnel, the tunnel will be run at a slower speed than when small mold blocks are used. Nevertheless, efficiency of the machine may be greater for the large molds since a greater weight of-material may be processed using the cooling capacity of the machine efficiently. For the greater speeds and lesser weights of material processed in small molds, problems arise in utilising the cooling capacity fully.
Further problems may arise in the provision of carrier means for the mold blocks suitable for use with very large mold blocks and for small mold blocks.
It has frequently been found desirable to utilise different travelling mold tunnels for molding large diameter tube and small diameter tube resulting, for some users in the capital expense of more than one machine.